Low distortion signal reproduction apparatus



Nov. 24, 1970 CHEN SA WANG 3,542,952

LOW DIS'I'ORTION SIGNAL REPRODUCTION APPARATUS Filed May 18, 1967 2 Sheets-Sheet 1 INVENTOR Chien San Won FIG .4,

BY JAM, ,Mfizd ATTORNEYS Nov. 24, 1 970 CHIEN SAN WANG LOW DISTORTION SIGNAL REPRODUCTION APPARATUS Filed May 18, 1967 RIO cl 2 Sheets-Sheet 2 C-TO -v couvenrsn FIG.5.

AMPLIFIER mvsmox Chi en Son Wong A'BY ATTORNEYS 3,542,952 LOW DISTORTION SIGNAL REPRODUCTION APPARATUS Chien San Wang, 1201 Hudson, Denver, Colo. 80220 Filed May 18, 1967, Ser. No. 639,534 Int. Cl. H04r 3/00 US. Cl. 179-1 8 Claims ABSTRACT OF THE DISCLOSURE A low distortion signal or sound reproduction apparatus including a transistorized zero junction circuit used to detect the current flow to a speaker or other output transducer and provide a negative feedback voltage which is linearly related to said current flow. The feedback voltage is used to control the signal applied to the input of the reproduction apparatus resulting in a low distortion current being applied to said device,

With the present state of the art in sound or signal reproduction, the only serious problems remaining for true reproduction of sound is the distortion of the CUR- RENT output from the amplifier to the speaker or other transducer load caused by the changing electrical impedance of the speaker (commonly used, permanent magnet moving coil cone speaker). It is well known that the electrical impedance of subject speakers will vary with such parameters as current level, frequency, motion amplitude, and the basic speaker design (permanent magnet and coil size).

Assuming a perfect voltage feedback amplifier, it is impossible to correct the distortion of the current fiow caused by changing speaker impedances. Prior art amplifier systems can not sense current distortion because the output voltage signal will be the same as the input voltage signal even though radical distortion of the current flow to the speaker is occurring. However, speakers are current controlled devices and cone displacement is due to the force caused by the current. This problem of speaker induced current distortion has long been recognized, but until now there has been no adequate solution.

The desired solution is to be able to sense the current going to the speaker (without introducing any loading and distorting device into the system) and then converting this current through proper transformation to a voltage that can be used as feedback. This voltage should have the exact wave form as the current Wave form with no phase shift. Hence, the amplifier would now be forced to produce a current that will be independent of impedance changes in the speaker and will be proportional to the input voltage signal. The preceding definition can properly be called a current source amplifier. Until now, this device has never been achieved.

Therefore, it is an object of this invention to provide a new and improved signal reproduction system that reduces current distortion in the signal output.

It is a further object of this invention to provide a new and improved sound reproduction system that has reduced distortion in the audio output caused by impedance changes in the speaker.

It is yet another object of this invention to provide a new and improved signal reproduction system that has reduced current distortion in the signal output caused by impedance changes in the load connected to the output of the reproduction system.

It will be appreciated that a system of the type desired by the invention requires the inclusion of a current-tovoltage conversion system. It will also be appreciated that prior art current-to-voltage conversion systems have nited States Patent ice normally added undesirable additional resistance to the current side of the conversion system. The addition of resistance has decreased the sensitivity of the conversion system and increased the error in the output signal. That is, the addition of resistance materially changes the characteristic of the signal being detected. Hence, it is desirable to provide a sensing system that will not introduce either voltage or resistance to the circuit whose current signal is being sensed.

Therefore, it is another object of this invention to provide a new and improved signal reproduction system that reduces distortion in the output of the system without adding undesirable additional resistance or voltage to the output from the system.

SUMMARY OF THE INVENTION In accordance with a principle of this invention, a novel current-to-voltage conversion system is used in a signal reproduction system to convert current fluctuations into a voltage fluctuations signal that is appliedto the signal input of the reproduction system. In operation, a current sensing loop is connected to a novel zero junction circuit wherein the current fluctuations are transformed into voltage fluctuations of proportional magnitude.

A zero junction circuit has two points in its circuitry where the voltage difference is insignificantly small even under changing current flow. The presence of this junction to any outside circuit connected to these two points does not affect the current flow in the outside circuitry. This device therefore provides a junction able to sense current summations of a circuit that has been connected to the zero junction points without any adverse effect on the circuit. In fact, many outside circuits can be connected to these zero-junction points for a total summation of all of the current flowing in all of the outside circuits. In accordance with the invention the current summations are then transferred to a voltage amplifying loop which completes the circuitry. Hence, a current-tovoltage transformation takes place without modifying the original monitored signal.

In accordance with another principle of the invention, the zero junction circuit is composed of two active elements (transistors). The first element (transistor) senses the current differential and the second element together with the first provides a zero junction; this provides a non-loading active junction with the capability of achieving current summation (algebraically). The current flow differential is sensed by the zero junction circuit to cause an identical current flow in the voltage gain loop of the first element. The current flow in this loop across a resistance provides a voltage linearly related with the current differential sensed by the zero junction circuit.

In accordance with but still another principle of the invention, the current-to-voltage conversion network is connected to sense loud speaker current transformed to a proper magnitude. This transformed current is fed to the zero junction circuit and converted to a voltage of proper magnitude which is used as a feedback voltage to the high gain operational power amplifier forming the signal reproduction system. An inherent characteristic obtained with an amplifier having a resistive feedback network is that the feedback voltage has the same wave form and has an amplitude that is proportional to the signal voltage applied to the input of the amplifier. Hence, the current fed to the loud speaker has the same wave form and proportional amplitude as the input signal voltage applied to the amplifier.

The foregoing objects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a common base transistor circuit used to explain a portion of the invention;

FIG. 2 is a schematic diagram of a current bucking reference and sensing loops used to explain a part of the invention;

FIG. 3 is a schematic diagram of two transistors used to explain the zero junction aspect of the invention;

FIG. 4 is a schematic diagram illustrating one embodiment of the current-to-voltage conversion system used in the invention;

FIG. 5 is a partially schematic and partially blocked diagram illustrating a sound reproduction current compensating system made in accordance with the invention; and

FIG. 6 is a schematic diagram illustrating how the current-to-voltage conversion system of the invention is connected in a sound reproduction system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Prior to discussing the preferred embodiment of the invention, a description of the current-to-voltage system used by the invention is provided.

Turning now to the drawings wherein like reference numerals designate like parts throughout the several views, FIG. 1 is a schematic diagram of a common base transistor circuit and comprises a first NPN (or PNP properly used) transistor Q1, an emitter resistor R1, a collector resistor R2, a first voltage source V1, and a second voltage source V2.

The emitter of Q1 is connected through R1 in series with V1 to the base of Q1 so that the emitter-base junction is forward biased. The collector of transistor Q1 is connected through R2 in series with V2 to the base of Q1 so that the collector-base junction is reverse biased.

As is well known in the transistor art, the current in the base-collector loop of a common base circuit will track the current in the base-emitter loop. That is, the current conversion factor or a of a common base circuit for most transistors is near unity. Hence, if the current in the base-emitter loop is made to vary the current in the base-collector loop follows it in a very near linear manner and because the current in that loop tracks the current in the emitter-base loop, the voltage drop across R2 tracks the current in the emitter-base loop. Further, by making R2 much, much larger than R1 and V2 larger than V1 a large voltage drop occurs across R2. This R2 voltage drop is then the major voltage drop in the collector-base loop. Hence, the common base circuit provides a near linear current-to-voltage conversion. Further, by making V2 and R2 large, a small change in emitter-base current causes a large change in the voltage drop across R2.

FIG. 2 is a schematic diagram of a current reference loop and a current sensing loop connected in bucking relationship. The circuit illustrated in FIG. 2 comprises a third voltage source V3, a fourth voltage source V4, a thermistor T and the emitter-base resistor R1 of FIG. 1. Resistor R1 is connected in series with the third voltage source V3 between a pair of points A and B. A wire connects points A and B so that a current reference loop is formed. Also connected across points A and B is the thermistor T in series with the fourth voltage source V4. The wire connecting points A and B is common to T and V4 so that a thermistor sensing loop is formed. The voltage sources V3 and V4 are connected so that they apply opposite polarity voltages to points A and B.

It will be appreciated by those skilled in the art that for a particular value of the thermistors resistance the values of R1, V3 and V4 can be chosen so that the voltage across points A and B is zero. When the voltage across points A and B is zero the current flow through the wire connecting these points is also zero, i.e. a balance condition exists. Thereafter if the resistance of the thermistor T increases or decreases an unbalance condition occurs and a current flows in one direction or the other through the wire connecting points A and B. This current flow is the difference between the current flow in the reference loop comprising V3, R1, and points A and B and the current in the sensing loop comprising T, V4, and points A and B. That is, FIG. 2 illustrates a simple current bucking network wherein the current through a wire connecting points A and B is the difference between the currents in the two loops connected to those points.

It should be noted that a circuit of the type illustrated in FIG. 2 is considerably more accurate for measuring than is a single loop circuit. Specifically, if the wire connecting points A and B is removed and an ideal meter of no resistance is placed across these points, the meter is much more sensitive to fluctuations caused by variations in the resistance of the thermistor, than is the meter if placed in a loop comprising a meter, the thermistor T, and the voltage source V4. More specifically, the meter can be adjusted so that full scale deflection is caused by the fluctuating current as opposed to full scale deflection being caused by a steady state current plus the fluctuating current.

FIG. 3 is a simplified schematic diagram illustrating two transistors connected to provide zero voltage across their emitter terminals. The circuit illustrated in FIG. 3 comprises a first NPN transistor Q1 and a second NPN transistor Q2. The first transistor Q1 is illustrated as having its emitter connected to point A and the second transistor Q2 is illustrated as having its emitter connected to point B. The bases of the transistors are connected together and to a common wire. For purposes of clarity, the collectors of the transistors are not illustrated.

By connecting the common base wire to a positive voltage source and by suitably adjusting the transistors biasing resistors (not shown) a near equal amount of current flow will pass through the transistors. Reasonable changes of current that would produce an unbalance in the base-emitter flow of Q1 and Q2, will not affect the near zero-voltage differential between points A and B.

As will be hereinafter described, this transistor network can be connected across points A and B of FIG. 2 to sense the unbalance current. More specifically, a variation in current flow through the transistors is a variation in the emitter-base current. This emitter-base current variation is sensed in a common base circuit of the type illustrated in FIG. 1. Hence, the dual transistors of FIG. 3 provide a zero junction network. Further, the common base conversion circuit in FIG. 1 provides isolation between the sensing emitter-base loop and the collectorbase loop.

Turning now to FIG. 4, which illustrates the foregoing relationship; that is, FIG. 4 illustrates the systems described in FIGS. 1, 2 and 3 connected together.

The system illustrated in FIG. 4 comprises the pair of NPN transistors Q1 and Q2, the reference loop comprising resistor R1 and voltage source V3, the thermistor loop comprising thermistor T and voltage source V4, the junction between R1 and T or point A connected to the emitter of Q1, and the junction between V3 and V4 or point B connected to the emitter of Q2. The bases of Q1 and Q2 are connected together and through a third resistor R3 to the positive side of the third voltage source V3. The collector of Q1 is connected to the base of Q1 through the series circuit consisting of the second voltage source V2 and the second resistor R2. Finally, the collector of Q2 is connected to the base of Q2, through a fourth resistor R4. A pair of output terminals are connected across the voltage sensing resistor R2.

The circuit illustrated in FIG. 4 combines the opera tions of the circuits illustrated in FIGS. 1, 2 and 3 to provide a current-to-voltage conversion system. That is, the components of the circuit are picked so that the voltage across points A to B is near zero for any desired resistance value of the thermistor T to provide an initial or steady state condition.

While there is zero voltage across points A and B, there is current flowing through the transistors due to the bias voltages provided by the various voltage sources. The bias current through the emitter-base loop of Q1 creates a current in its collector-base loop to develop an initial or steady state voltage across the output terminals.

When the thermistors resistance varies the current flow through the emitter-base junction of Q1 varies. As described with respect to FIG. 1, the variation in the emitterbase current flow of Q1 varies the current in Qls collector-base circuit. This latter current variation varies the voltage drop across R2 thereby varying the voltage across output terminals. Hence, the overall system provides a current-to-voltage conversion. Further, by making V2 and R2 large a very small current fluctuation through the emitter-base circuit of Q1 provides a large voltage fluctuation across R2. That is, the circuit components can be chosen so that a micro or milliamp current change through the emitter-base circuit of Q1 provides a change across R2 measured in magnitude of several volts.

It will be appreciated by those skilled in the art and others that the output voltage of the embodiment of the invention illustrated in FIG. 4 is equal to steady state voltage E plus a voltage change AE caused by a change in the current flowing through the emitter base circuit of Q1.

FIG. 5 and FIG. 6 illustrate a preferred embodiment of the invention wherein the current-to-voltage conversion system is used to sense the current passing through a speaker coil and generates a feedback voltage to be ap plied to the speakers amplifier. The system illustrated in FIG. 5 comprises an amplifier A, a speaker S, a toroidal transformer T1, the current-to-voltage conversion system X, a first capacitor C1, a tenth resistor R10, a second capacitor C2, and an eleventh resistor R11.

The output of the amplifier A is connected to the primary winding of T1 in series with the speakers winding. The secondary winding of T1 is connected to the input of the current-to-voltage conversion system X. The transformer takes the place of the thermistor illustrated in FIG. 4. The thermistor was therein used to simplify the description of the basic operation of the current-to-voltage conversion system used by the preferred embodiment of the invention. The output of the current-to-voltage conversion system X is connected through the tenth resistor R10 in series with the first capacitor C1 across the input of the amplifier A. A coupling network comprising C2 and R11 is illustrated as connecting a pair of input terminals to the amplifier A.

Preferably, the current transformer transforms high current levels to small usable current levels in the range of 0-10 ma.

The current-to-voltage conversion system X illustrated in FIG. 6 comprises the first and second NPN transistors Q1 and Q2, first, second, third, fourth, fifth and sixth resistors R12, R13, R14, R15, R16 and R17, and a voltage source V6. R12 equals R13 for the preferred operation of the system. The emitter of the first transistor is connected to point A and through the first resistor R12 to the negative terminal of the voltage source V6. Similarly, the emitter of the second transistor Q2 is connected to point B and through the second resistor R13 to the negative side of V6. The third and fourth resistors R14 and R15 are connected in series across the voltage source V6 and the junction between the third and fourth resistors is connected to the bases of the first and second transistors. The fifth resistor R16 is connected between the collector of Q1 and the positive side of V6 and the sixth resistor R17 is connected between the collector of Q2 and the positive side of V6.

The secondary of the transformer is applied across the points A and B, which comprise the points of the zero junction circuitry. Hence, the transformer is electrically shorted in its output. It is a well known fact that the output of a current transformer must be shorted in order to produce a near perfect current transformation of the wave form seen in the primary windings. Also, under these circumstances there is no phase shift. Hence, current must be sensed or detected with a zero junction circuit in order that the secondary will be short circuited and proper performance given. This allows a monitoring of the current flow in the same fashion as the zero junction circuit has worked in the previous explanation.

In operation, the transformer T1 senses current flow patterns in the signal applied to the speaker 5. These current flow patterns are transformed by the current-tovoltage conversion system X in the manner hereinabove described to voltage patterns that duplicate proportionally the current flow patterns. These voltage flow patterns are connected through the capacitor C1 and resistor R10 to the input of the amplifier A to correct for the distortions in the current flow pattern produced by the speaker impedance changes. That is, the voltage fluctuations applied to the input of the amplifier operate to negate any distortion in the current flow pattern thereby preventing distortion in the speakers audio output. Hence, the system improves the performance of the speaker in an audio sound system by eliminating current flow pattern distortion caused by changing speaker impedance.

The foregoing has described a preferred embodiment of the invention. That is, FIG. 5 illustrates an apparatus for improving the speaker performance of an audio system by using the current source amplifier previously described. Hence, the electrical impedance of the speaker can change in any known way without affecting the current-force relationship needed to produce high-fidelity sound. In summary, this invention eliminates impedance change as a distorting factor, something which has not been achieved in prior art sound (or more generally signal) reproduction systems.

Further, in light of the foregoing disclosure of the invention, it should be apparent to a person skilled in the art that the subject matter of this invention can be used to algebraically sum two or more AC signals without cross modulation. For example, in making a recording, a signal carrying a singers voice can be combined with a signal carrying the orchestra all without cross modulation.

The low distortion signal reproduction apparatus of the invention can be utilized in other types of signal or audio sound systems. Moreover, the invention can be used with force coil operating devices in other than audio sound systems such as recording instrument systems, for example. This invention is particularly suitable in environments where the current is low but the desired voltage must be high. Because a high voltage output from a small current input is achieved by the invention, unsophisticated amplifying devices can be used to amplify the voltage changes to even high voltages and/or power levels, if necessary. That is, many prior art current-tovoltage conversion systems change milliamp changes to millivolt changes, for example. However, this invention provides a means for changing a micro or milliamp change to full volt changes. Hence, the conversion ratio is considerably improved. Because of this direct transformation to a relatively high voltage, amplification of the output voltage can be more easily performed.

What is claimed is:

1. A low distortion audio sound reproduction system COIIlPI'lSlIlgI a speaker connected to the output of said amplifier;

a current sensing loop including a current sensing means connected between said amplifier and said speaker for sensing the current fluctuations in the signal applied by said amplifier to said speaker;

current-to-voltage conversion means connected to said current sensing means for converting said current fluctuations into voltage fluctuations, said current to voltage conversion means including a zero junction circuit connected to said current sensing loop for detecting the current flow in said current sensing loop and for generating a voltage output in accordance with said current flow; and,

coupling means connecting the output of said current-to-voltage conversion means to the input of said amplifier.

2. Apparatus as claimed in claim 1 wherein said zero junction means includes a first transistor and a second transistor, said first transistor converting said current fluctuations to said voltage output and said secondtransistor balancing said first transistor to provide a zero junction.

3. Apparatus as claimed in claim 2 wherein said current sensing means is a toroidal current transformer having its secondary winding connected to said zero junction circuit.

4. A low distortion audio sound reproduction system comprising:

an amplifier;

a speaker connected to the output of said amplifier;

a current sensing means connected between said amplifier and said speaker for sensing the current fluctuations in the signal applied by said amplifier to said speaker;

current-to-voltage conversion means connected to said current sensing means for converting said current fluctuations to voltage fluctuations, said current-tovoltage conversion means comprising:

first and second transistors;

first, second, third fourth, fifth and sixth resistors;

a voltage source;

the emitters of said first and second transistors connected to the output of said current sensing means;

the emitter of said first transistor connected through said first resistor to one terminal of said voltage source and the emitter of said second transistor connected through said second resistor to the same terminal of said voltage source;

said third and fourth resistor connected in series across said voltage source with the junction between said third and fourth resistors connected to the bases of said first and second transistors;

said fifth resistor connected between the collector of said first transistor and the second terminal of said voltage source; and,

said sixth resistor connected between the collector of said second transistor and the second terminal of said voltage source; and

coupling means connected across sixth resistor and to the input of said amplifier for coupling said current-to-voltage conversion means to the input of said amplifier. r

5. Apparatus as claimed in claim 4 wherein said current sensing means is a toroidal current transformer having its secondary winding connected to the emitters of said first and second transistors.

6. A low distortion system comprising:

an amplifier means for amplifying an electrical signal;

a current sensing loop including a current sensing means connected to the output of said amplifier means for sensing current fluctuations in the output of said amplifier means;

current-to-voltage conversion means connected to said current sensing means for converting said current fluctuations into voltage fluctuations, said currentto-voltage conversion means including a zero junction circuit connected to said current sensing loop for detecting the current flow in said current sensing loop and for generating a voltage output in accordance with said current flow; and,

coupling means connecting the output of said currentto-voltage conversion means to the input of said amplifier means.

7. Apparatus as claimed in claim 6 wherein said zero junction means includes a first transistor and a second transistor, said first transistor converting said current fluctuations to said voltage output and said second transistor balancing said first transistor to provide a zero junction.

8. A low distortion system comprising:

an amplifier means for amplifying an electrical signal;

a current sensing means connected to the output of said amplifier means for sensing current fluctuations in the output of said amplifier means; current-to-voltage conversion means connected to said current sensing means for converting said current fluctuations to voltage fluctuations, said current-tovoltage conversion means comprising:

first and second transistors; first, second, third, fourth, fifth and sixth resistors; a voltage source; the emitters of said first and second transistors connected to the output of said current sensing means; the emitter of said first transistor connected through said first resistor to one terminal of said voltage source and the emitter of said second transistor connected through said second resistor to the same terminal of said voltage source; the third and fourth resistors connectedin series across said voltage source with the junction between said third and fourth resistors connected to the bases of said first and second transistors; said fifth resistor connected between the collector of said first transistor and the second terminal of said voltage source; and, said sixth resistor connected between the collector of said second transistor and the second terminal of said voltage source; and, a coupling means connected across said sixth resistor and to the input of said amplifier means for coupling said current-to-voltage conversion means to the input of said amplifier means.

References Cited UNITED STATES PATENTS 2,905,761 9/1959 Wilkins 179-l 3,073,899 1/1963 Famsworth 179-1 3,408,575 10/1968 Petrocelli et a1 1791X KATHLEEN H. OLAFFY, Primary Examiner C. W. JIRAUCH, Assistant Examiner US. Cl. X.R. 

